The structure and properties of aqueous lipid mixtures analogous to bile in various states of cholesterol saturation will be studied by combined ESR spin label techniques and molecular sieve chromatography. The spin label technique will be used to study the molecular organization within the mixed aggregates and their stability in bulk solution. Molecular sieve chromatography will be employed to characterize the number of aggregated species, their populations, lipid stoichiometry and mechanisms of cholesterol precipitation from supersaturated solutions. These methods will be applied to study lipid structures and equilibration phenomena in bile from animals and humans.